1. Field of the Invention
This invention relates to a data carrier having a semiconductor chip in which secret data are stored. The invention relates in particular to a smart card.
2. Description of Related Art
Data carriers containing chips are used in a great number of different applications, for example for performing monetary transactions, paying for goods or services or as identification means for access or admission controls. In all such applications the chip of the data carrier normally processes secret data which must be protected from access by unauthorized third parties. Such protection is ensured by, among other things, giving the inner structures of the chip very small dimensions so that it is very difficult to access said structures with the aim of spying out data processed in said structures. In order to impede access further, the chip can be embedded in a very firmly adhering mass whose forcible removal destroys the semiconductor plate or at least the secret data stored therein. It is likewise possible to provide the semiconductor plate during its production with a protective layer which cannot be removed without destroying the semiconductor plate.
With corresponding technical equipment, which is extremely expensive but nevertheless fundamentally available, an attacker could possibly succeed in exposing and examining the inner structure of the chip. Exposure could be effected for example by special etching methods or a suitable grinding process. The thus exposed structures of the chip, such as conductive paths, could be contacted with microprobes or examined by other methods to determine the signal patterns in said structures. Subsequently one could attempt to determine from the detected signals secret data of the data carrier, such as secret keys, in order to use them for purposes of manipulation. One could likewise attempt to selectively influence the signal patterns in the exposed structures via the microprobes.
U.S. Pat. No. 4,932,053 discloses a data carrier with semiconductor chips which has at least one memory in which an operating program containing a plurality of commands is stored. Each command causes signals detectable from outside the semiconductor chip. The signals are measured by current consumption at the terminals of the integrated circuit, permitting the processed data to be inferred. To prevent reading, a protection circuit is provided which generates a pseudorandom sequence by means of simulation cells. The current behavior which is measurable from outside is thus superimposed with a random signal.
French laid-open print FR-A-2 745 924 discloses making signals unrecognizable by using a random number generator which leads to desynchronization during execution of instruction sequences or program sequences within the processor.